Electrical insulators for high voltage use have traditionally been made of glass or porcelain as these materials are of low cost, high quality electrically and under normal conditions, have a long life in service. However, at higher voltages, the size and weight of glass or porcelain becomes excessive. A number of composite insulators made from lighter weight polymeric materials have been developed for use in such high voltage installations. Such composite insulators generally include a fiberglas rod having a number of weathersheds constructed of a highly insulating polymeric material attached to the rod along its length.
Since the 1970's, advancements in high voltage insulator technology have been made by introducing new, lightweight polymer materials for the manufacture of insulators—replacing the heavier and more brittle porcelain and glass materials conventionally used in the manufacture of insulators. These new polymer materials include Silicone, EPDM, ESP, EPR, Bisphenol Epoxy and Cycloaliphatic Epoxy polymers.
Although they offer the advantages of reduced weight and mechanical flexibility, insulators made from any of these polymers have demonstrated failure and inconsistent performance when subjected to normal field phenomena such as environmental weathering, UV exposure, exposure to industrial pollution, exposure to salt fog and salt spray, electrical tracking, corona discharge and electrical arcing. Unlike this new technology, the older porcelain/glass technology is extremely reliable. Insulators made of porcelain/glass have a minimum lifespan of 35 years compared to inconsistent lifespans of only 2 to 20 years with polymer-based composite insulators.
The primary modes of failure of polymer-based composite insulators are erosion of the polymer material, splitting/cracking of the polymer material and brittle fracture of the supporting fiberglass core (due to the interface of dissimilar materials; polymer and fiberglass). Loss of hydrophobicity is also a common mode of failure, particularly in insulators made of non-silicone polymers. These non-silicone polymer-based composite insulators have thus not gained much acceptance in the market, despite their substantially lower cost compared to silicone polymer-based composite insulators and porcelain/glass insulators.
An insulator such as the suspension insulator in a high voltage power transmission line is designed to keep to a minimum current discharges under normal conditions. However, when the insulator surface becomes contaminated, leakage current can develop along the surface of the insulator. The amount of this leakage current depends upon the voltage stress and the conductivity of the film of contaminant on the surface of the insulator. These leakage currents can incur or cause arcing on the surface of the insulator which can have serious effects upon the composite insulator surface such as the formation of free carbon and non volatile semiconductor materials and may eventually result in a conducting path forming across the surface of the insulator effectively shorting out the insulator.
The outer surface of an electrical insulator is the most important part of the insulator as this is the part of the insulator that is subjected to the effects of electrical voltage stress, leakage currents and weathering. When the surface of a high voltage insulator is exposed to moisture such as rain or fog in combination with contaminated atmospheres as are found in industrial locations may be subject to extensive corrosion unless protected in some way from exposure to the corrosive atmosphere. Other potentially corrosive environments include along sea coasts where salt spray is found and in areas where agricultural chemicals are widely distributed.
The room temperature curable silicone composition of the present invention used to coat the outer surface of insulators provides for improved insulation that is arc resistant, hydrophobic and resistant to the stresses imposed upon outdoor electrical insulator. The composition provides a coating of electrically non conductive material on the surface of the insulator which protects the underlying insulator material.